Child Soldiers International said young recruits had left education early and were "actively sought" for more dangerous infantry roles.

The charity urged the Government to raise the minimum enlistment age to 18 in a letter to defence minister Penny Mordaunt, which is co-signed by the general secretary of the National Union of Teachers Christine Blower and the children's commissioners for Scotland, Wales and Northern Ireland.

The enlistment age should be raised to 18, campaigners say

The Ministry of Defence said the military provided a "challenging and constructive education" for young people, who could only join under the age of 18 with consent of their parents or guardians.

The letter said: "Most of the world now recognises that the recruitment of children for military purposes is detrimental to their best interests and is not appropriate in modern armed forces.

"In British society, the law safeguards children by proscribing choices that entail significant risks until they turn 18, the legal age of majority. It is therefore incongruous that the British armed forces still enlist personnel at age 16 and accept applications from age 15.

"Premature enlistment can also bring significant risk and disadvantage."

The charity said official figures showed around one third of 16 and 17-year-olds who join the military leave during their training. This meant they faced "elevated risks" of unemployment and mental health problems as a result, having already left education to join the armed forces.

And those that stayed could face a higher risk in combat roles once they are 18, with many joining the infantry.

The Government could save £51 million a year in training costs by raising the enlistment age, the charity suggested.

The letter, which is co-signed by 20 bodies including Amnesty International and Unicef UK, said: "We recognise that many young people may wish to choose a career in the armed forces, but in view of the risks and legal obligations involved, the choice to enlist should be fully informed and only made once the young people have reached the age of legal majority."

Rachel Taylor, programme manager at Child Soldiers International, said: "The Ministry of Defence claims that targeting disadvantaged and often vulnerable young people for premature enlistment is in their best interests.

"Children's rights experts at the UN and across the UK are unanimous in stating that it is not. We urge the minister to listen to and respect the expertise of these specialists."

"Enlisting minors is an outdated practice. There are better ways for the MoD to engage with young people interested in a military career, which better protect their welfare in both the short and long term."

Soldiers must serve at least four years in the Army but those who join when they are under 18 must serve a minimum of four years from their 18th birthday, something Child Soldiers International claimed "would be unlawful in civilian life".

A Ministry of Defence spokesman said: " As part of our duty of care to our recruits no young person under the age of 18 years may join our armed forces without the formal written consent of their parent or guardian, nor do they deploy on operations until they reach 18 years of age.

"We take pride in the fact our armed forces provide challenging and constructive education, training, and employment opportunities for young people, equipping them with valuable skills."